Fault code B1421 – a step-by-step guide to diagnosing and fixing it

Error code B1421 is displayed and the check engine light is on - what does this mean?

The DTC B1421 is recognized by the OBD/EOBD system and indicates a fault in the body control module (BCM). This trouble code mainly affects the body's electrical systems such as lighting, power windows or central locking. The specific meaning of the error code can vary depending on the manufacturer.

What causes can this error code have?

• Defective body control module (BCM)
• Broken or damaged cable connections
• Loose or corroded plug connections
• Defective fuses in the body area
• Faulty earth connections
• Short circuit in the wiring
• Defective sensors or actuators in the body system
• Voltage problems in the on-board power supply

What symptoms occur in parallel with fault code B1421?

• Lighting problems: Headlights, tail lights or interior lighting do not work properly
• Window regulator faults: Electric windows do not respond or operate erratically
• Central locking fault: Doors cannot be locked or unlocked centrally
• Warning lights: Various indicator lights in the dashboard illuminate continuously
• Air conditioning problems: Heating or air conditioning not working correctly
• Radio/infotainment failure: Electronic systems in the interior do not work

How to fix the problem of fault code B1421?

• Repairing fault code B1421 requires a systematic diagnosis of the body electrics.
• Carry out a complete OBD diagnosis and read out all stored fault codes, then check body control module for function and replace if defective
• Check all electrical connections to the BCM, clean damaged or corroded plugs and repair or replace defective cables
• Check fuses in the fuse box and replace blown fuses, also checking the corresponding circuits for short circuits
• Locate ground connections on the body and check that they are tight and free of corrosion, clean and tighten loose or corroded ground points
• Measure the vehicle electrical system voltage and test the battery and alternator for proper function, replace the relevant components if there are voltage problems
• Carry out a software update for the body control module or have the control unit reprogrammed